1. Field of Invention
This invention is concerned with altering the vertical blanking interval of a video sync signal in accordance with supplied information, and, more particularly, with altering the post-equalizing portion thereof.
2. Description of the Prior Art
A standard television composite signal includes video intervals alternating with blanking intervals. In amplitude, the video intervals vary in amplitude within the video level on one side of (i.e., above or below -- positive going or negative going) a fixed amplitude level often known as the blanking level or the black level, while synchronization pulses during the blanking interval extend to an amplitude known as the sync level on the other side of (below or above -- negative going or positive going) the blanking level. The sync level is also known as the blacker-than-black level. For purposes of discussion, the video signal will be assumed to be above the blanking level and the sync will be assumed to be below the blanking level.
There are two types of blanking intervals, the horizontal blanking interval which occurs once for each line of the video image and which contains a single horizontal sync pulse, and the vertical blanking interval which occurs after each field (3121/2 or 2621/2 lines) of video information. The vertical blanking interval includes six relatively wide vertical synchronization pulses preceded by six relatively narrow pre-equalization pulses and followed by six relatively narrow post-equalization pulses all extending between the blanking level and sync level. The vertical blanking interval also contains other sync pulses which are like the horizontal sync pulses in duration and spacing.
Television engineering professionals have, for years, tried to expand the information content of the composite signal to add features not contemplated by the originators of the signal specification. Many of the ideas for expanding the information contents of the signal in the vertical blanking interval involve placing information in the video amplitude levels. Some practitioners place information in the video amplitude portion of the vertical blanking interval where the horizontal rate sync pulses are located with the idea that the information is located in a portion of the signal which is at the top of the TV signal and thus not observable by a viewer.
U.S. Pat. Nos. 4,625,235 and 4,638,359, issued Nov. 25, 1986 and Jan. 20, 1987, respectively, to John N. Watson describe and illustrate the alteration of the post-equalization pulse portion of the vertical blanking interval to add information content as illustrated in FIG. 6A. In FIG. 6A the vertical axis represents amplitude and the horizontal axis represents time. As illustrated in FIG. 6A, the video level is up, positive going, and the sync pulse level is down, negative going, relative to the unlegended blanking level.
In the prior art, all modification to the signal, except for color burst, have involved changing the video amplitudes and have not involved modifying the sync signals. The color burst is located in the horizontal blanking interval following the horizontal sync pulse in the portion of the horizontal blanking interval known as the "back porch". It varies in amplitude like the video signal, but unlike the video signal, is centered about the blanking level.
The actual duration of the various sync pulses has not been altered in the prior art. In the early days of television, the timing and duration of each horizontal sync pulse was considered unalterable and there were no synchronization pulses in the vertical blanking interval. It was soon realized that with no synchronizing pulses in the vertical blanking interval that the TV receiver would not stay synchronized with the transmitting equipment. The solution was to add synchronization pulses in the vertical blanking interval, particularly, the pre-equalization and vertical sync pulse interval pulses. The pre-equalizing pulses were found to be necessary due to the interlaced nature of the standard television signal. Without the pre-equalizing pulses the position of the horizontal pulses relative to the vertical pulses would shift by half line every field. The placement of the pre-equalization pulses solved that problem.
The six post-equalizing pulses were added, however, for a different reason. It was found that vacuum-tube blocking oscillators used in early TV sets were upset in some manner if the sync returned directly to the horizontal pulses after the vertical sync pulses were received. The six post-equalizing pulses were therefore added to protect what was then a universally used pulse identification circuit. The six post-equalizing pulses do not currently serve a useful purpose, but they may not be deleted because they are part of more than a half-century old standard.